Traders on the floor of the New York Stock Exchange on Tuesday. / Richard Drew, AP

by Adam Shell, USA TODAY

by Adam Shell, USA TODAY

NEW YORK -- The Dow Jones industrial average closed above 15,000 for the first time ever on Monday, marking the latest milestone in a powerful bull market that has had few true believers since it began in March 2009.

Gaining 0.6% for the day -- 87 points -- the Dow settled at 15,056.20. The S&P 500, which gained 0.5%, also had a record close, hitting 1,625.96.

While reaching the new milestone certainly has psychological implications for investors, it also is a time for them to reevaluate their investments and whether they have the right mix of assets, says Erik Davidson, deputy chief investment officer at Wells Fargo Private Bank.

"Like any milestone event in life, like having a baby, celebrating a birthday or graduating from college, it's a reason to pause and reflect," says Davidson.

Dow 15,000 could finally convince investors that have been on the sidelines earning roughly 0% on their cash to redeploy some assets into the stock market, says Joseph Tanious, global market strategist at J.P. Morgan Funds.

"The fact that the market is breaking out to new highs is getting more people off the sidelines," which is reflected in fund-flow data to U.S.-focused stock mutual funds, says Tanious.

He believes the market has more upside, perhaps as much as 5%, thanks to the Federal Reserve's easy-money policies, an earnings season in which worst-case fears have not been realized and signs that the job market and economy continue to gradually improve.

The market's ability to keep going up, however, is raising the question of how long it can last.

Bur Farr adds that there is "reason for caution" because the market is more expensive today from a price-to-earnings standpoint than it was when the rally began four years ago. "The market's getting pricier," says Farr, adding that the market's P-E multiple has risen from 12 at the market trough to roughly 14 or 15 in the current bull market.

Rob Lutts, chief investment officer at Cabot Money Management, says investors are reacting to a slow but steady improvement in the U.S. economy and he expects more gains ahead. According to his calculations, the market is 15% undervalued, Lutts says.

"The basic underlying foundation of the economy is a lot stronger than many people believe," says Lutts, adding that the economy is getting a big boost from the rebounding housing market and the nation's increasing push to become more energy efficient.

The fact that many investors, both on Main Street and institutional investors, remain underweight in stocks is also a plus, as it means the market should continue to get fresh flows of cash as investors look to take part in the rally and exit conservative assets that are yielding little if anything at all.

Lutts balks at talk that the market's new highs are a sign that it can't rise much more.

"The stock market is essentially back where it was 12 or 13 years ago," he says. "And when you look at it that way, you could have some good catch-up in performance."

Overseas Tuesday, Japanese stocks outperformed all others as traders returned from a public holiday in buoyant mood, sending the Nikkei stock index above 14,000 for the first time in nearly five years.

The Nikkei surged 3.6% to 14,180.24 on its first day of trading following the Golden Week holiday - that's the first time the Nikkei has breached the 14,000 mark since June 2008.

Japanese stocks have been marked up heavily after the Bank of Japan announced a new aggressive monetary policy to get the country out of its near two-decade stagnation. One repercussion of the plan to pump huge amounts of yen into the Japanese economy has been to sharply weaken the currency. A lower yen is boosting economic growth by making the country's exports cheaper in international markets.

Elsewhere, investors remained largely positive amid a dearth of scheduled economic and corporate news, as they continued to draw encouragement from Friday's better than anticipated U.S. payrolls figures. The data often set the market tone for a week or two after their release.

In Europe, the FTSE 100 index of leading British shares as up 0.3% at 6,540 while Germany's DAX rose 0.6% to 8,164. The CAC-40 in France was 0.4% higher at 3,921.

"Wall Street may be eyeing a relatively unchanged start to Tuesday, but there's no denying the current levels remain bullish and the fact the S&P is holding above 1,600 is certainly worthy of note," said Fawad Razaqzada, market strategist at GFT Markets.

The dollar was also fairly steady after gaining in the wake of the payrolls data. The euro was flat at $1.3075 while the dollar was 0.2% lower at 99.19 yen.

Australia was in focus after the Reserve Bank of Australia lowered its official interest rate by a quarter percentage point to 2.75% amid some signs the economy is coming off the boil as the Australian dollar rises. Following the reduction, the Aussie dollar fell 0.9% to $1.0169. However, the S&P/ASX 200 stock index fell 0.2% to 5,143.70.

Oil prices drifted lower after a strong run, with the benchmark New York rate down 71 cents at $95.45 a barrel.

On Monday, U.S. stocks ended mixed. The Dow fell less than 0.1% to 14,968.89. The S&P 500 rose 0.2% to 1,617.50. The Nasdaq composite index rose 0.4% to 3,392.97.